Horrible MTN service

Bought a Samsung phone from their Newmarket store a week ago and unfortunately the phone started acting up on the 7day evening of the guaranteed exchange period. I now only have the option of getting it repaired. I don't want a phone that already needs repairs within the first week of using it, what other problems lie ahead for me. I bought this for work and repairs mean 2 weeks minimum without working, then I will have a repaired phone. This is where customer service and customer care kick in especially because the phone was checked at the store, it's not damaged in anyway by me and they can't figure out why the sensor stopped working. I DON'T WANT A REPAIRED PROBLEMATIC PHONE. I paid full price for a new phone. If I wanted a repaired phone I would have gone to the Pakistanis around the corner. How is this supposed to be acceptable from such a big south African company. What's the point of buying new, but demo, new? Try other service providers if you can. I'm so furious